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  Differential amplifier and method for generating computer simulation model thereofThe Patent Description & Claims data below is from USPTO Patent Application 20060226906. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to amplifier technology, and more particularly to a differential amplifier which is thermally stable by adopting a ballast resistor to a differential structure of a unit transistor pair, such that the differential amplifier prevents heat effect phenomena, such as performance deterioration and device destruction by heating, and, at the same time, improves or maintains other performance traits, thereby achieving high gain, high efficiency, high linearity, and wide bandwidth characteristics, and a method for generating a computer simulation model of the differential amplifier. [0003] 2. Description of the Related Art [0004] Generally, an amplifier is designed to obtain high output power and high efficiency. When high output power is an objective, a large amount of heat is inevitably generated inside an amplifier, such that the heat causes performance deterioration or destruction of unit transistors. [0005] Especially, since bipolar transistors have a thermal characteristic of positive feedback, in a power amplifier designed by connecting a plurality of transistors, a large amount of heat is generated at the middle portion, in which transistors are more tightly packed, than at the edge. Also, since turn-on voltage at a relatively high temperature is lowered due to positive feedback and larger currents are generated in the middle of the power amplifier, a large amount of heat is generated therein. [0006] As such a feedback is repeated, temperature of the transistors in the middle of the power amplifier is increased, and a large amount of current flows therethrough, such that the power amplifier suffers complete failure, finally resulting in complete destruction of the transistors. [0007] In order to prevent such an outcome, each unit transistor forming an amplifier employs a ballast resistor to have negative feedback characteristics, such that thermal equilibrium can be maintained among unit cells, thereby protecting the transistors from destruction. [0008] FIG. 1 is a view illustrating a circuit of an amplifier according to the prior art, in which each unit cell 15 includes a transistor 13 and a ballast resistor 14, which is connected to the emitter of the transistor 13, and thus forms thermally negative feedback. Therefore, the amplifier prevents heat-induced performance deterioration and device destruction. [0009] However, since such a ballast resistor 14, employed in each cell to form thermally negative feedback, forms electrically negative feedback as well as thermally negative feedback, gain and efficiency of the amplifier are decreased. [0010] FIG. 2 is a view illustrating a circuit of an amplifier according to another embodiment of the prior art, in which each unit cell 27 includes a unit transistor 24, a capacitor 26 connected to the base of the transistor 24 to transfer an RF signal to the base and a ballast resistor 25 located between a node, which is positioned between the capacitor 26 and the base, and DC voltage lead 22. Therefore, the amplifier prevents loss of an RF signal and is thermally stable through the DC bias path including the ballast resistor 25. [0011] However, the prior art amplifier has disadvantages in that frequency bandwidth of the amplifier is decreased by such a capacitor, which is serially connected to the base to separate the RF signal path from the DC bias path, and also, its size is enlarged due to the capacitors. SUMMARY OF THE INVENTION [0012] Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a differential amplifier which is thermally stable by adopting a ballast resistor to a differential structure of a unit transistor pair, such that the differential amplifier prevents heat effect phenomena, such as performance deterioration and device destruction, and, at the same time, improves or maintains other performances, thereby achieving high gain, high efficiency, high linearity, and wide bandwidth characteristics, and a method for generating a computer simulation model of the differential amplifier. [0013] In accordance with the present invention, the above and other objects can be accomplished by the provision of a differential amplifier comprising a plurality of unit cells which are connected to each other in parallel, wherein each unit cell includes a unit transistor pair, which is formed as a differential structure, and a ballast resistor which is connected to common emitter of the unit transistor pair. [0014] Preferably, the device inputs differential inputs and outputs differential outputs. [0015] In accordance with another aspect of the present invention, there is provided a method for generating a computer simulation model of a differential amplifier comprising a plurality of unit cells which are connected to each other in parallel, wherein the unit cell includes a unit transistor pair, which is formed as a differential structure, and a ballast resistor which is connected to common emitter of the unit transistor pair. The method comprising the steps of: extracting DC characteristics and thermal characteristics based on a common mode pattern which is formed through interconnection of bases of the unit cells and interconnection of collectors of the unit cells; extracting electrical characteristics by a half amplifier pattern which is formed as one side differential transistors of unit cells and emitter ballast resistors are connected to each other in parallel; extracting emitter resistance and parasitic components by common emitter ballast resistor pattern which is formed as emitter ballast resistors of the unit cells are connected to each other in parallel; and modeling a differential amplifier as a single unit cell based on the extracted values which are obtained through the common mode pattern, the half amplifier pattern, and the common emitter ballast resistor pattern. [0016] Preferably, the unit common emitter ballast resistance of the unit cell is extracted from the unit common emitter ballast resistor pattern, and the total common emitter ballast resistance is derived by dividing the unit common emitter ballast resistor by the number of unit cells. [0017] The present invention, constructed as mentioned above, is thermally stable by adoption of a ballast resistor to a differential structure of a unit transistor pair, such that the differential amplifier prevents heat effect phenomena, such as performance deterioration and device destruction by heating, and, at the same time, improves or maintains other performances, thereby achieving high gain, high efficiency, high linearity, and wide bandwidth characteristics. Also, the present invention provides a computer simulation model enabling precise design of the differential amplifier. BRIEF DESCRIPTION OF THE DRAWINGS [0018] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: [0019] FIG. 1 is a view illustrating a circuit of an amplifier according to the prior art; [0020] FIG. 2 is a view illustrating a circuit of an amplifier according to another embodiment of the prior art; [0021] FIG. 3 is a view illustrating a circuit of a differential amplifier according to the present invention; Continue reading... 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